This invention relates to a construction of a fuel assembly spacer used for various nuclear power reactors of the type where the fuel assembly is inserted in a coolant duct and, more particularly, to a fuel assembly spacer which has a plurality of springs having a unit for spring strength and arranged around the outer periphery of the spacer so that the fuel assembly can be positioned coaxially with the coolant duct.
In the design of a nuclear power reactor, in which the heat produced from the nuclear fuel is transmitted to a coolant, as the criterion for evaluation of the soundness of the fuel at the present it is necessary to provide sufficient heat removal capacity lest burn-out of fuel should result. Considerations in this respect should also be given to the design of the spacer. The basic function of the spacer is to keep the gap between adjacent fuel rods at a prescribed value so as to prevent formation of hot spots, either thermal or radioactive. Also, it should not unduly interfere with the flow of the coolant contributing to the removal and transmission of heat from the fuel, and also it is required to have sufficient strength to reliably hold the fuel rods in their normal positions even against vibrations due to the flow force of the coolant, thermal bending due to non-uniform thermal distribution in the fuel rods and other stresses due to external loads.
However, results of recent research reveals that the performance of removal of heat from the fuel by the coolant is greatly influenced by such factor as the contour of the fuel assembly including the spacers. Particularly, where a channel tube or a pressure tube is used as an outer structure of a fuel assembly constituting a duct for the passage of a coolant, there is some tolerance in the cross-sectional demension of the coolant duct during its manufacture, and also the sectional area of the duct tends to increase due to the secular creep deformation caused by stresses produced during use of the reactor and by irradiation with neutrons. Therefore, the effects of the positional contour of the fuel assembly within the coolant duct during the thermal performance of the fuel cannot be ignored.
However, the design of the conventional spacers has been made mainly from the standpoint of the structural strength, and the aforementioned effects of the contour of the fuel assembly including the spacers during the heat removal performance have not been taken into detailed consideration.